In optical communication, a pilot signal is a function signal that is loaded on a high-rate data optical signal and used for a function such as supervision, control, equalization, synchronization, or reference. For an Orthogonal Frequency Division Multiplexing (OFDM) optical signal including a plurality of subcarriers, a pilot signal may be loaded on the OFDM optical signal in a process of forming the OFDM optical signal. In design of an OFDM optical signal, a quantity of subcarriers in the OFDM optical signal is determined according to a specific situation of a baseband signal, and before the OFDM optical signal is formed, which subcarriers in all subcarriers of the OFDM optical signals are used for loading which baseband signals are predetermined.
In the prior art, during modulation of an OFDM optical signal, in a case in which not all subcarriers are loaded with baseband signals, subcarriers that are not loaded with baseband signals are used for loading pilot signals. A specific process is as follows: First, in the OFDM optical signal, a subcarrier used for loading a pilot signal is determined from all the subcarriers that are not loaded with baseband signals, and bit-to-symbol mapping is performed on a baseband signal to map a bit sequence of the baseband signal to a symbol sequence; then serial-to-parallel conversion is performed on the symbol sequence of the baseband signal to divide the symbol sequence of the baseband signal into groups according to a quantity of values included in each symbol and obtain a plurality of sub symbol sequences, where each sub symbol sequence corresponds to a subcarrier for loading the symbol sequence; and for the subcarrier used for loading the pilot signal, a sub symbol sequence corresponding to the subcarrier is set to a constant value. Then Inverse Discrete Fourier Transform (IDFT) is performed on each group of sub symbol sequences corresponding to the baseband signal and the pilot signal, so that values of the sub symbol sequences in a frequency domain are transformed to a time domain. Then a cyclic prefix is inserted in the sub symbol sequences in the time domain, and parallel-to-serial conversion is performed to obtain a service signal time domain sequence that is formed by all the sub symbol sequences and loaded on all the subcarriers. Then clipping and digital-to-analog conversion are performed, an OFDM electrical signal loaded with the pilot signal is formed, and electrical amplification is performed on the OFDM electrical signal loaded with the pilot signal. Finally, the OFDM electrical signal loaded with the pilot signal and a light wave are modulated into an OFDM optical signal loaded with the pilot signal.
During implementation of the present invention, the inventor finds that the prior art has at least the following problem:
In a case in which not all subcarriers are loaded with baseband signals, when subcarriers that are not loaded with baseband signals are used for loading pilot signals, a quantity of remaining subcarriers that are not loaded with baseband signals is limited. Therefore, a quantity of pilot signals to be loaded is generally greater than a quantity of subcarriers that are not loaded with baseband signals, and an implementation environment of the prior art is limited. Consequently, the pilot signal loading manner provided by the prior art has a certain limitation.